A projection display apparatus is widely used to produces an enlarged image of a small-sized micro-display through a projection optical system (or a projection lens). The small-sized micro-display includes for example a high temperature liquid crystal display (HTLCD), a digital micro-minor display (DMD) or a liquid crystal on silicon (LCoS).
Generally, a wide-angle projection lens has a large field of view, or a short effective focal length (EFL). In comparison with a conventional projection lens, the projection display apparatus with a wide-angle projection lens is capable of producing a certain sized image at a shorter distance.
Recently, an integrated system of a projection display apparatus and an interactive white board has become very useful tool in classrooms, lecture rooms or conference rooms in order to provide the interactive functions about education, demonstration or entertainment. FIG. 1A is a schematic diagram illustrating an integrated system of a projection display apparatus and an interactive white board according to the prior art. As shown in FIG. 1A, the projection display apparatus 10 is usually mounted upside down over the white board 11. The conventional short-throw projection display apparatus 10 is usually mounted at a distance of about 1 meter away from the white board 11. As such, the lecturer with average height becomes an obstacle to the light path of the projection display apparatus 10 when writing on the white board 11. In addition, the eyes of the lecturer have the potential danger of being illuminated by the light from the projection display apparatus 10.
For solving the above problems, as shown in FIG. 1B, the projection display apparatus 10 needs to be mounted in the vicinity of the white board 11. Since the projection distance is very short, the projection display apparatus 10 should have a wide-angle projection lens to provide a large full field angle θ or a very short focal length. In a case that the projection display apparatus 10 with a wide-angle projection lens is mounted over the white board 11, the light flux needs to incident on the white board 11 at a very steep angle, which incurs a large distortion in the image. Moreover, the projection lens also needs to have a large offset to avoid the light hitting on the main body of the projection display apparatus, and avoid the body of reflecting mirror blocking the top area of the white board. In other words, the wide-angle or ultra-short projection lens is very critical for designing the projection display apparatus.
U.S. Pat. Nos. 6,631,994 and 6,994,437 disclosed a lens design that incorporates negative lenses and a negative aspheric mirror to achieve a wide field of view. In this design, negative lenses with moderate power are used with the negative reflecting mirror. With such an arrangement of optical power, it is difficult to simultaneously achieve very large full field angle and high image quality at the same time.
U.S. Pat. No. 7,529,032 disclosed a wide-angle projection optical system. The design uses two aspheric plastic lenses, one double concave negative lens and a negative aspheric reflecting mirror. The total length of the system is quite long. This makes it difficult to have a thin projector system. In addition, the maximum field angle is on the order of 55 degree, which still requires a fairly long projection distance to produce an image that is large enough for practical applications.
U.S. Pat. No. 7,239,452 disclosed a wide-angle projection lens which uses aspheric lenses and reflecting mirror. However, one flat mirror is disposed between the negative lens group and the negative aspheric mirror. This type of optical path layout makes it suitable mostly for rear projection application, not convenient for front projection applications.
U.S. Pat. No. 6,771,427 disclosed a wide angle projection lens design that incorporates regular negative lens group and an aspheric reflecting mirror on the image side of the lens layout. Due to the use of regular spherical lenses, the full field angle of this design is quite small, and multiple flat reflecting mirrors are needed to achieve thin profile.
U.S. Pat. No. 7,048,388 disclosed a wide-angle projection lens design that incorporates a flat mirror and a positive aspheric reflecting mirror. Again, this type of layout tends to make the projector bulky and inconvenient to use with a white board.
U.S. Pat. No. 7,448,756 disclosed a wide-angle lens design that consists of a series of negative lenses and a negative reflecting mirror. In this design, an intermediate real image with a specific distortion is formed in the light path and is then re-imaged on to the screen by the rest of the optical system. Due to the formation of an intermediate real image, the optical path of this design tends to be long and complicated, which make it difficult to have a compact and thin display system.